Hi Dale. I don’t really think your critique is substantiated if you can’t read past the title of the article, regardless I will entertain your argument for a moment.
Firstly, you’re right in saying that many of the mass slaughters that have occurred in recent history have taken place in the past 100 years. But rather than assume that this is because humans have become more bloodthirsty, I would contend that the greater volume of people living today coupled with the evolution in weaponry are the more likely culprits.
Secondly, your point on there being no evidence that pre-agricultural societies engaged in warfare is simply incorrect. Numerous archaeological records (see here and here for example) now indicate that violence was as, if not more prevalent in hunter-gatherer times on a per-capita basis.